Methanol is a major chemical raw material. Present global consumption is about 27 million tons per year. Major uses of methanol include the production of acetic acid, formaldehyde, and methy-t-butylether. The latter, an oxygenate additive to gasoline, accounts for about a third of all use. Worldwide demand for methanol is expected to increase as much as five fold over the next decade as potential new applications become commercialized. Such applications include the conversion of methanol to gasoline, the conversion of methanol to light olefins, the use of methanol for power generation, and the use of methanol for fuel-cell powered automobiles.
In general, methanol synthesis is based on the equilibrium reactions of synthesis gas (often abbreviated to “syngas”), namely reactions (1) and (2):CO+2H2⇄CH3OH   (1)CO2+3H2⇄CH3OH+H2O   (2)
Synthesis gas is defined as a gas comprising primarily carbon monoxide (CO), carbon dioxide (CO2) and hydrogen (H2). Other gases present in synthesis gas typically include methane (CH4), and small amounts of light paraffins, such as ethane and propane. One way of characterizing the composition of a synthesis gas stream for the production of methanol is to account for the CO2 present in the synthesis gas stream. The syngas number (SN) is defined as follows:SN═(H2—CO2)/(CO+CO2)
The forward reactions (1) and (2) are exothermic, that is, they result in the formation of net heat. Also, the forward reactions (1) and (2) generate less volumes of MeOH (gas) than the volumes of feed (gas) used to form the methanol. Therefore, to maximize methanol yields, i.e., force reactions (1) and (2) to the right, the process requires low temperatures and high pressures for high conversion. Still, a typical methanol reactor will convert only about 20% to 60% of the synthesis gas fed to the reactor in a single pass through. To obtain higher conversions the unreacted synthesis gas is separated from the product methanol and recycled back to the reactor or directed to a second reactor to produce additional methanol.
The initial step in the production of methanol is to produce synthesis gas from a hydrocarbon-containing feed, typically a methane-containing feed, such as natural gas or refinery off-gas. The associated costs of producing the synthesis gas currently account for over half of the capital investment in the methanol plant. Known methods for producing synthesis gas include steam reforming, non-catalytic and catalytic partial oxidation and autothermal reforming.
In a steam reforming process, steam is reacted with a hydrocarbon containing feed over a catalyst, such as nickel, nickel oxide, cobalt oxide, chromia and/or molybdenum oxide, to produce a hydrogen-rich synthesis gas. The general stoichiometry, as illustrated for methane, is:CH4+H2O⇄CO+3H2   (3)Typically, an excess of steam is used to drive the equilibrium to the right.
Because of the high endothermicity of the reaction, steam reforming is typically carried out in large furnaces, in which the catalyst is packed into tubes. The tubes must withstand the high pressure of the produced synthesis gas, while transmitting heat at temperatures approaching 1000° C. As described in Stanford Research Institute International Report No 148A (1995), steam reforming process efficiency, defined as the heat of combustion of product synthesis gas divided by the heat of combustion of reforming feed and furnace fuel, is approximately 79%, while the space velocity, defined as Standard Cubic Feet per Hour of C1-equivalent feed/ft3 of catalyst bed is 690 hr−1. Unfortunately, steam reforming furnaces occupy a very large volume of space, orders of magnitude greater than the tube volume, such that low productivity limits the economic attractiveness of the process.
The partial oxidation process involves the partial oxidation of a hydrocarbon containing feed in the gas phase. The process can be carried out with or without a catalyst. In non-catalytic partial oxidation the feed components are introduced to a burner where they combust with sub-stoichiometric oxygen to produce a synthesis gas mixture. Catalytic partial oxidation comprises passing a gaseous hydrocarbon mixture, and oxygen, preferably in the form of air, over a reduced or unreduced composite catalyst, conveniently containing one or more transition metals. The ideal partial oxidation reaction, as illustrated for methane, is:CH4+½O2⇄CO+2H2   (4)
However, gas-phase reaction kinetics tend to over-oxidize some of the feed, resulting in excessive heat generation and substantial yield of H2O, CO2, and unreacted hydrocarbons that may leave the reactor as soot. For these reasons, when gas phase partial oxidation chemistry is applied to clean feeds, it is preferred to add steam to the feed and add a bed of steam reforming catalyst to the bottom of the gas phase partial oxidation reactor vessel. This combination of gas phase partial oxidation and steam reforming is called autothermal reforming. The heat needed for steam reforming is provided in-situ from the excess heat generated by the gas phase partial oxidation reactions.
Autothermal reforming can be substantially more compact than steam reforming because furnaces are not used to provide heat of reaction. Embodiments of autothermal reforming presently under development for gasoline-powered fuel cell vehicles make use of a catalytic partial oxidation step, which can impart even greater compactness to the autothermal reforming process. The thermal efficiency of autothermal reforming reactors are generally in the range of 90%. While autothermal reforming based on catalytic partial oxidation may have high productivity (gas hourly space velocity on order of 104 as C1-equivalent), conventional partial oxidation-based autothermal reforming has a space velocity very similar to that of steam reforming (about 103 hr−1). However, autothermal reforming requires a source of oxygen. In the fuel cell vehicle case, this oxygen in typically provided as low-pressure air, which results in a nitrogen-diluted, low-pressure synthesis gas. In refinery or chemicals embodiments, this oxygen is typically provided as purified O2, but the cost of air separation can be greater than the cost of the autothermal reforming process.
U.S. Pat. Nos. 4,200,682, 4,240,805, 4,293,315, 4,642,272 and 4,816,353 to Sederquist teach a steam reforming process in which the heat of reforming is provided within the catalyst bed by cycling between combustion and reforming stages of a cycle. As described by Sederquist, the high quality of heat recovery within the reforming bed results in a theoretical efficiency of about 97%. However, the examples and commercial projections within these patents describe a process that operates at very low productivity, with space velocities of around 95 hr−1 (as C1-equivalent). Moreover, this process requires a compressor to compress the product synthesis gas to useful pressures.
Copending U.S. patent application Ser. No. 10/458,399 filed Jun. 10, 2003 describes an improved cyclic steam reforming process for producing synthesis gas. In this process, the reforming stage involves preheating a first zone to a temperature in the range of about 700° C. to 2000° C. and then introducing a 20° C. to 600° C. hydrocarbon-containing feed, along with steam and optionally CO2 to the inlet of the first zone. The hydrocarbon is reformed into synthesis gas over a catalyst in the first zone and the synthesis gas is then passed to a second zone, where the synthesis gas is cooled to a temperature close to the inlet temperature of the hydrocarbon feed and is recovered. In the regeneration stage, an oxygen-containing gas and fuel are combusted near the interface of the two zones, producing a hot flue gas that travels across the first zone, thus re-heating that zone to a temperature high enough to reform the feed. Once heat regeneration is completed, the cycle is completed and reforming begins again. An advantage of this process is the ability to operate the reforming stage at a higher pressure than the regeneration stage, thus creating a pressure swing, and producing high pressure synthesis gas.
The present invention provides a process scheme for integrating a cyclic reforming process for producing synthesis gas, such as that disclosed U.S. patent application Ser. No. 10/458,399, with a process for converting the synthesis gas to methanol. The process scheme seeks to effect this integration in a manner that maximizes thermal efficiency and minimizes capital cost.